Magnetic Surface Effects and Magnetoresistance in Manganite-based Composite Nanoparticles

摘要

The effect of Ag doping on the magnetization and the magneto transport of (1-x)La_(0.7)Ca_(0.3)MnO_3/xAg composite nanoparticles have been studied. The introduction of Ag into the grain boundaries forms a second conducting network and improves conductivity and increases the metal-insulator transition temperature. The magnetoresistance of composites in the whole studied temperature range is enhanced by increasing the Ag content. The enhanced extrinsic magnetoresistance can be related with the spin dependent scattering of spin-polarized electrons at the interfaces between the La_(0.7)Ca_(0.3)MnO_3 grains and silver granules; whereas the monotonic increase in the intrinsic magnetoresistance has been attributed to such factors like: decrease in electrical resistivity due to the disorder reduction, magnetic heterogeneity, and growth of spin clusters. Both high field magnetoresistance (HFMR) and low field magnetoresistance (LFMR) have been calculated as a function of temperature. The characteristic of HFMR and LFMR turns out to be substantially different: While the LFMR decreases rapidly with temperature, the HFMR is increased slightly with temperature. We propose that for the Ag-doped nanomanganites, the HFMR originates from the weakened ferromagnetic spin interaction at the grain boundaries. In addition, it is shown that these composites become magnetically harder when reducing the Ag dopant. It is believed that the phenomena are due to the effect of Ag on the degree of the spin disorder at the grain boundaries. Implications of these finding improvements of the field response of the magnetic surface of nanoparticles are discussed.